Wind-induced dust generation and transport mechanics on a bare agricultural field

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Abstract

Strong atmospheric winds may cause wind erosion and dust emissions on bare, dry, erodible fields. Since these dust emissions may exceed particulate matter limits established by the United States Environmental Protection Agency, information on dust generation and transport mechanics is needed to determine the particulate hazard of dust sources. Measurements of climatic variables and airborne sediment mass and concentration were made during three strong wind events on a bare, fine sandy loam field in west Texas. This study clearly shows that dust flux estimates were very sensitive to dust concentration measurement height. PM10 flux values estimated between heights of 2 and 5 m were 2–5 times those estimated between heights of 5 and 10 m. Tower placement in relation to the upwind unerodible boundary produced significant differences in dust flux that varied with storm intensity. During the most intense storm event, the PM10 flux between heights of 2 and 5 m measured at the tower 200 m from the unerodible boundary was almost 2.5 times as that measured at the tower 100 m from the unerodible boundary. Vertical PM10 dust flux was closely related with horizontal sediment flux only when the winds came from the same direction during the entire duration of horizontal sediment flux measurements.

Introduction

The Clean Air Act, amended in 1990, required the US Environmental Protection Agency (USEPA) to establish National Ambient Air Quality Standards (NAAQS). These standards set limits on airborne pollutants, including particulate matter, considered harmful to the public and the environment. The standards were designed to protect public health and welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings [1]. Particulate matter is subdivided by size. Particles with a mass median aerodynamic diameter of less than 10 μm are called PM10 and particles with a mass median aerodynamic diameter of less than 2.5 μm are called PM2.5. PM10 particles pose health risks because they can be inhaled into the respiratory system and the PM2.5 pose a greater risk because they can be inhaled deeply into the lungs. Although many pollutants originate from industrial and other anthropogenic sources, geologic materials may contribute significant airborne particulate matter.

Much is known and has been written about wind as a geological process causing aeolian sediment transport and deposition of particulate matter [2], [3], [4], [5], [6]. Interest continues in this topic as shown by recent conferences in Africa and West Asia [7], Europe [8] and the United States [9]. Airborne particles originating from geologic materials can have many sources and may pose threats to humans and animals, depending upon the size and geochemistry of the particles and any materials adsorbed onto the particles. In this paper, we limit our discussion to the emission and transport of suspended particles (or fugitive dust) from earth surfaces due to the force of the wind, a process often called wind erosion. Although sources of suspended dust are numerous and varied, similar processes occur when dust is emitted from deserts, dry lake beds, agricultural fields, dirt roads, construction sites, and other areas where the surface is bare and erodible particles are exposed to the force of winds.

Particles moved by the wind can range up to about 1 mm in diameter, but particles travelling great distances are usually much smaller (<100 μm). Particles of fine dust (<20 μm) have a low settling velocity, even under low wind speeds, and may be transported great distances and kept suspended in the atmosphere for a very long time [10]. Wind erosion is a significant source of fine dust and PM10, particularly in regions of highly erodible soils [11], [12], [13].

Field studies of airborne dust produced at or near the origin of intense dust sources are difficult to conduct yet numerous studies have been reported [14], [15], [16], [17], [18], [19], [20], [21], [22]. Most studies have focused on total suspended dust <20 μm. However, due to the interest in PM10 in the NAAQS, recent studies in the US have focused on PM10 emissions [20], [21], [22], [23], [24].

Fine dust is generally emitted due to the force of saltating particles impacting the soil surface [6], [14], [25]. Recent work in silty loessial soils of the US Pacific Northwest suggests that fine dust may also be entrained into the atmosphere due to the direct force of the wind, without saltation bombardment [24]. Work by Gillette et al. [20] has related horizontal mass flux of sediment to the vertical flux of PM10 particles for a large sandy playa lake in California. The lake represents an unusual large eroding surface. Information on the vertical flux of PM10 particles is needed to determine the potential particulate hazard of eroding surfaces. Since the USEPA designated the southern half of the Owens Valley as a ‘Serious’ PM10 non-attainment area, a State Implementation Plan was developed that calls for the control of dust on 43 km2 of the lake bed [23].

However, most eroding surfaces are often much smaller than the Owens Lake bed and the study of suspended dust poses significant challenges. If the field is very small, the total amount of emitted dust may be determined by simply measuring the vertical profile of dust concentrations of the plume and multiplying by the wind speed to obtain a horizontal flux. However, this may not be practical if the field is so large that the entire plume cannot be sampled or estimated or if there are many heterogeneous source areas in large fields.

Many studies use the gradient method, described by Gillette [26], to estimate vertical flux of suspended dust. The application of the gradient method in agricultural fields is not clear. The method requires measurement of dust at two heights and seems to assume a fully developed dust plume, yet no studies have described the effect of dust sensors height or placement in relation to a developing dust plume close to the dust source on vertical dust flux measurements. In addition, agricultural fields are often so variable or small that horizontal emissions are not uniform. In this paper, we test the hypothesis that large variations in vertical dust flux may arise depending upon sensor and tower placement in a small agricultural field. In addition, we describe the effect of horizontal mass flux on vertical dust flux in small fields and demonstrate the importance of wind direction and sampler placement.

Section snippets

Experimental site

The study site was located in the southern Great Plains of west Texas at the United States Department of Agriculture, Agricultural Research Service (USDA-ARS), Wind Erosion and Water Conservation Research Unit field station in Big Spring, Texas (32.2702N, 101.4865W). The climate is semiarid with a mean annual temperature of 17.1 °C, mean annual precipitation of 470 mm and mean annual wind speed of 8 m s−1. The study was conducted on an Amarillo fine sandy loam (13% clay, 78% sand and 0.3% organic

Results and discussion

A summary of the wind profile characteristics determined for periods when saltation was active and dust concentration measurements were collected is listed in Table 1. Dust concentration measurements were collected in the late morning and early afternoon on each day.

The observation periods ranged from 240 to 395 min long. The wind originated from the west–southwest (252–258° mean wind direction) on all days. The mean 2 m wind speeds were lower than the mean threshold wind speeds during the same

Conclusions

Estimates of vertical dust flux are often obtained by measuring dust concentrations at two heights and then applying a diffusion equation similar to Eq. (3). No standard heights at which to make dust concentration measurements are often specified. In cases where the suspended dust is thoroughly mixed with a uniform concentration near the surface in the atmospheric boundary layer, specification of measurement heights may not be needed. This was the case for the west tower on March 4. Eroding

Acknowledgements

The authors are grateful for the diligent efforts provided by Ace Berry, Charles Yates and James Davis for site installation, data collection and processing assistance. The authors also thank the anonymous reviewers for their valuable assistance in improving this manuscript.

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